FIG. 1 is a schematic of a voltage generator 100 in accordance with the prior art (U.S. Pat. No. 10,007,289 B2). The voltage generator 100 comprises a current mirror 102, two n-type transistors 104, 106, and a resistor 108. The current mirror 102 comprises two p-type transistors 110, 112.
The p-type transistor 110 has a source coupled to a supply voltage VDD, a drain coupled to a drain of the n-type transistor 106, and a gate coupled to a gate and a drain of the p-type transistor 112. The p-type transistor 112 has a source coupled to the supply voltage VDD.
The n-type transistor 106 has a source coupled to ground and has a gate coupled to its drain and a gate of the n-type transistor 104. The n-type transistor 104 has a drain coupled to the drain of the p-type transistor 112 and a source coupled to a first terminal of the resistor 108 at an output node NA. A second terminal of the resistor 108 is coupled to ground. An output voltage Vo is provided at the output node NA.
The transistors 104, 106, 108, 110 are metal oxide semiconductor field effect transistors (MOSFET), and as such p-type transistors may be referred to as PMOS, and n-type transistors may be referred to as NMOS.
The n-type transistors 104, 106 have different threshold voltages, and specifically, the threshold voltage of the n-type transistor 106 is greater than the threshold voltage of n-type transistor 104. The output voltage Vo is equal to the difference between the threshold voltages of the n-type transistors 104, 106. The difference between the threshold voltages may be equal to the bandgap voltage of silicon.
The output voltage Vo may be used as a reference voltage for use in a different part of a circuit implementing the voltage generator 100 or may be provided to another circuit for use as a reference voltage.
Operation of the voltage generator 100 requires that a minimum operation voltage is provided. The operation voltage is the voltage difference between the supply voltage VDD and ground, and as such, in the voltage generator 100, the minimum operation voltage necessary to operate the voltage generator 100 corresponds to a minimum supply voltage VDD.
The minimum operation voltage is dependent on the threshold voltages of the n-type transistors 104, 106 and the minimum operation voltage may be lowered by reducing the threshold voltages of the n-type transistors 104, 106.
Additionally, the minimum operation voltage is dependent on the series coupling of the p-type transistor 112, the n-type transistor 104 and the resistor 108 from the supply voltage VDD to ground. The voltage from the source of the p-type transistor 112 to ground is equal to the sum of the voltage across the source and drain (the source-to-drain voltage) of the p-type transistor 112, the voltage across the drain and source (the drain-to-source voltage) of the n-type transistor 104 and the output voltage Vo. The supply voltage VDD must be sufficiently large such that the portion of the supply voltage VDD provided across the drain and source of the n-type transistor 104 is sufficiently large to operate the n-type transistor 104 in its saturation mode.
In the case that the output voltage Vo is approximately equal to the bandgap voltage of silicon, as in the voltage generator 100, the output voltage Vo is approximately equal to 1.2V and the minimum operation voltage may be greater than or equal to 2V in common technology. As the output voltage Vo is dependent on the bandgap voltage of silicon, the minimum operation voltage cannot be decreased below approximately 2V by reducing the threshold voltage of the n-type transistors 104, 106.
The threshold voltage is the minimum voltage across the gate and source (the gate-to-source voltage) of a transistor that permits a current to flow between the source and drain terminals of the transistor. As the drain of the p-type transistor 112 is coupled to its gate, the source-to-drain voltage of the p-type transistor 112 is limited by the threshold voltage of the p-type transistor.
The threshold voltage of the p-type transistor 112 is greater than or equal to 0.6V in the voltage generator 100 for low current operation and to avoid the influence of leakage current. The source-to-drain voltage of the p-type transistor 112 must be equal to threshold voltage to permit current flow. Therefore the source-to-drain voltage may be approximately equal to or greater than 0.6V. Therefore, the requirements for the source-to-drain voltage of the p-type transistor 112 impacts the minimum operation voltage of the voltage generator 100.